Neurotrophic factors such as insulin-like growth factors, nerve growth factors, brain- derived neurotrophic factor, neurotrophin-3, -4/5 and -6, ciliary neurotrophic factor, GDNF, and neurturin have been proposed as potential means for enhancing specific neuronal cell survival, for example, as a treatment for neurodegenerative diseases such as amylotrophic lateral sclerosis, Alzheimer's disease, stroke, epilepsy, Huntington's disease, Parkinson's disease, and peripheral neuropathy. It would be desirable to provide additional therapy for this purpose. Protein neurotrophic factors, or neurotrophins, which influence growth and development of the vertebrate system, are believed to play an important role in promoting the differentiation, survival, and function of diverse groups of neurons in the brain and periphery. Neurotrophic factors are believed to have important signaling functions in neural tissues, based in part upon the precedent established with nerve growth factor (NGF). NGF supports the survival of sympathetic, sensory, and basal forebrain neurons both in vitro and in vivo. Administration of exogenous NGF rescues neurons from cell death during development. Conversely, removal or sequestration of endogenous NGF by administration of anti-NGF antibodies promotes such cell death (Heumann, J. Exp. Biol., 132:133–150 (1987); Hefti, J. Neurosci. 6:2155–2162 (1986); Thoenen, et al., Physiol. Rev., 60:1284–1335 (1980)).
Additional neurotrophic factors related to NGF have since been identified. These include brain-derived neurotrophic factor (BDNF) (Leibrock, et al., Nature, 341:149–152 (1989)); neurotrophin-3 (NT-3) (Kaisho, et al., FEBS Lett., 266:187 (1990); Maisonpierre, et al., Science, 247:1446 (1990); Rosenthal, et al., Neuron, 4:767 (1990), and neurotrophin 4/5 (NT-4/5) (Berkemeier, et al., Neuron, 7:857–866 (1991)).
Neurotrophins, similar to other polypeptide growth factors, affect their target cells through interactions with cell surface receptors. According to current understanding, two kinds of transmembrane glycoproteins act as receptors for the known neurotrophins. Equilibrium binding studies have shown that neurotrophin- responsive neuronal cells possess a common low molecular weight (65,000–80,000 Daltons), a low affinity receptor typically referred to as p75LNGFR or p75, and a high molecular weight (130,000–150,000 Dalton) receptor. The high affinity receptors are members of the trk family of receptor tyrosine kinases.
Receptor tyrosine kinases are known to serve as receptors for a variety of protein factors that promote cellular proliferation, differentiation, and survival. In addition to the trk receptors, examples of other receptor tyrosine kinases include the receptors for epidermal growth factor (EGF), fibroblast growth factor (FGF), and platelet-derived growth factor (PDGF). Typically, these receptors span the cell membrane, with one portion of the receptor being intracellular and in contact with the cytoplasm, and another portion of the receptor being extracellular. Binding of a ligand to the extracellular portion of the receptor induces tyrosine kinase activity in the intracellular portion of the receptor, with ensuing phosphorylation of various intracellular proteins involved in cellular signaling pathways.
Glial cell line derived neurotrophic factor (“GDNF”) and Neurturin (“NTN”) are two, recently identified, structurally related, potent survival factors for sympathetic sensory and central nervous system neurons (Lin et al. Science 260:1130–1132 (1993); Henderson et al. Science 266:1062–1064(1994); Buj-Bello et al., Neuron 15:821–828 (1995); Kotzbauer et al. Nature 384:467–470 (1996)). Recently, GDNF was shown to mediate its actions through a multi-component receptor system composed of a ligand binding glycosyl- phosphatidyl inositol (GPI) linked protein (designated GDNFRα; also designated GFR-α-1) and the transmembrane receptor tyrosine kinase Ret (Treanor et al. Nature 382:80–83 (1996); Jing et al. Cell 85:1113–1124 (1996); Trupp et al. Nature 381:785–789 (1996); Durbec et al. Nature 381:789–793 (1996)). The NTN signal is transmitted via GFRα2, which is also Ret associated.
Membrane-bound proteins and receptors can play an important role in the formation, differentiation and maintenance of multicellular organisms. The fate of many individual cells, e.g., proliferation, migration, differentiation, or interaction with other cells, is typically governed by information received from other cells and/or the immediate environment. This information is often transmitted by secreted polypeptides (for instance, mitogenic factors, survival factors, cytotoxic factors, differentiation factors, neuropeptides, and hormones) which are, in turn, received and interpreted by diverse cell receptors or membrane-bound proteins. Such membrane-bound proteins and cell receptors include, but are not limited to, cytokine receptors, receptor kinases, receptor phosphatases, receptors involved in cell—cell interactions, and cellular adhesin molecules like selectins and integrins. For instance, transduction of signals that regulate cell growth and differentiation is regulated in part by phosphorylation of various cellular proteins. Protein tyrosine kinases, enzymes that catalyze that process, can also act as growth factor receptors. Examples include fibroblast growth factor receptor and nerve growth factor receptor.
Membrane-bound proteins and receptor molecules have various industrial applications, including as pharmaceutical and diagnostic agents. Receptor immunoadhesins, for instance, can be employed as therapeutic agents to block receptor-ligand interaction. The membrane-bound proteins can also be employed for screening of potential peptide or small molecule inhibitors of the relevant receptor/ligand interaction.
The aberrant expression or uncontrolled regulation of any one of these receptor tyrosine kinases can result in different malignancies and pathological disorders. Therefore, there exists a need to identify means to regulate, control and manipulate receptor tyrosine kinases (“RTK”), their ligands, or their α-subunit receptor molecules, e.g., GPI-linked α-subunit receptors, to which they associate, in order to provide new and additional means for the diagnosis and therapy of receptor tyrosine kinase pathway-related disorders and cellular processes. The present application provides the clinician and researcher with such means by providing new molecules that are specific for interacting with certain receptor genes and their gene products. These compounds and their methods of use, as provided herein, allow exquisite therapeutic control and specificity. Accordingly, it is one object of the present invention to provide an improved therapy for the prevention and/or treatment of neurological conditions and other conditions in which certain neurotrophic signaling pathways play a role.